1. Field of the Invention
The present invention is related to measurement of lithographic exposure system parameters, and more particularly, to a dedicated metrology stage for lithography applications.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
The reticle is generally located between a semiconductor chip and a light source, usually mounted on a reticle stage. In photolithography, the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example. Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the silicon or semiconductor wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors, the key to creating more powerful microprocessors is the light's wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors results in a more powerful microprocessor.
A relatively common problem in the lithographic art is a need to measure parameters of the optical systems used for lithographic exposure. As a general rule, it is desirable to be able to do such measurements without taking the lithographic exposure system offline, and without disassembly and reassembly of components. The current practice in the industry is to place sensors on the wafer stage, to the extent space permits. These sensors are generally located in the space not occupied by the wafer itself, in the corners of the wafer stage.
However, with the ever increasing sophistication of exposure systems, with decreasing exposure wavelengths, and with increasing complexity of the optics, the number of different sensors that end users require is increasing. At the same time, there are severe constraints on the available space. For example, it is generally impractical or undesirable to increase the dimensions of the wafer stage, since this complicates stage positioning, and stage movement, and increases the dimensions of the overall lithographic equipment, which is problematic, since clean room space inside a fabrication facility is limited.
Accordingly, what is needed is a way to enable positioning of measurement sensors of lithographic exposure optics without affecting overall lithographic tool dimensions.